In logging while drilling (LWD) applications, it is advantageous to detect the presence of a formation anomaly ahead of a drill bit or around a bottom hole assembly.
U.S. patent application published under number 2006/0038571 describes methods for localizing an electromagnetic anomaly in a subterranean earth formation, employing transient electromagnetic methods. These methods particularly enable finding direction and distance from a transient electromagnetic measurement tool to a resistive or conductive anomaly in a formation surrounding a borehole in drilling applications.
In these methods, typically a tool, comprising a transmitter coil and a receiver coil, is lowered into a borehole in the earth formation. The transmitter coil produces a magnetic dipole field in the formation. Due to, for instance geometric properties of the transmitter system, in practice the dipole field will be an approximate dipole field. A transient response signal, comprising an induced voltage in the receiver coil, is measured after rapidly turning off the current that is passed through the transmitter antenna. The sudden drop is understood to generate decaying eddy currents in the formation, which in turn induce the transient response signal at the receiver antenna.
The referenced US patent application shows that relevant conductivity information of the earth formation is embodied in the response signals over the entire time span of the decay, starting already during the first microseconds after the sudden drop in the current and continuing up to perhaps even seconds.
The probing distance from the tool into the formation increases with length of the time span during which the decay is measurable. Thus, the minimum probing distance is limited by how fast the transmission can be terminated, and the maximum available probing distance is in practice limited by the fact that the response signals have decayed to such low values that they become impossible to measure with an acceptable signal-to-noise ratio.
This drives a need for increasing the signal relative to the noise, while maintaining an acceptable time resolution.